I am assuming you can make or buy a pinhole from a can or order one from a shop
Take your little flat head screwdriver and a well lit area
Locate the screw on the bottom of the ring on the lens housing and unscrew
Wriggle off the clack housing and put that, the. Ring and the little screw in a safe place.
Unscrew the glass the screw being on the right near the focus lever, then the glass lifts off easily. Polish this and store. Not required again
See the screws on the top and bottom?? Unscrew those.. Lever the shutter plate off gently BUT!!! ...
These wires need to be cut. The the plate will lift off. As this is my working normal clack, I will not do this...
You will be able to lift off this plate. You see the black plastic hole? this is where you glue your pinhole plate. Make sure it is centred and super glue (or tape if you wish to leave the option open
Now. Making sure your pinhole is secure. Reattach the shutter plate, and make sure the screws are seated properly. If not, your instant (1/30th sec) will not work. Reattach the cone and the ring.
If you love working with black & white, & are making a permanent change to the camera, you can install filters instead of the close up lens. Some people remove this, but it's handy to have.
Go out and be happy. Inbuilt filters for b&w and 1/30th sec for sunny days and 50asa 6X9 negatives. Happy pinhole Day!!!